1. Field of the Invention
The present invention relates to an image processing apparatus that processes input image data, an imaging apparatus that has functions of the image processing apparatus, an image processing method, and a computer program, and, more particularly to an image processing apparatus, an imaging apparatus, an image processing method, and a computer program suitable for processing of image data in which a scene having a relatively wide luminance range is imaged.
2. Description of the Related Art
With a digital still camera (DSC) employing a solid state imaging element, an image acquired by imaging can be recorded on a recording medium such as a flash memory as digital image data. The image can be displayed on a monitor or printed from a printer on the basis of the recorded digital image data.
During imaging in such a DSC, it is possible to obtain image data having a desired quality by applying AE (Auto Exposure) processing, AWB (Auto White Balance) processing, gradation correction processing, and the like to an imaged image in the DSC. When the AE processing is performed, a luminance of a subject is measured by, for example, a method of dividing an angle of view of the imaged image into plural areas and setting a weighted average luminance of luminances in the respective areas as the luminance of the subject or setting a luminance at a focus point as the luminance of the subject. An exposure amount is determined by adjusting an exposure time, an aperture of a stop, and an ISO (International Standards Organization) gain on the basis of the measured result.
However, in a scene having a wide luminance range in an angle of view, it is highly likely that the accuracy of the AE processing is deteriorated, a main subject in the angle of view is excessively exposed to light to cause white void in an image or is insufficiently exposed to light to cause drowning of an image in noise or black solid in the image. As an imaging method for obtaining an image imaged under an appropriate exposure condition even in such a scene, there is known a method called “bracket imaging” for exposing a subject to light plural times in a row by changing an exposure condition and obtaining plural image signals under the changed exposure conditions. For example, there is known an image photographing method for automatically setting appropriate conditions during bracket imaging on the basis of photometry results (see, for example, JP-A-2003-348438 (paragraphs 0047 to 0050 and FIG. 3).
However, even if the exposure condition is appropriately set, when a dynamic range of an output of an imaging element is insufficient with respect to a luminance range of the scene, white void or black solid occurs to deteriorate an image quality. In other words, in respective images obtained by the bracket imaging, it is difficult to reproduce illumination components exceeding the dynamic range of the output of the imaging element.
Therefore, an imaging method that makes it possible to obtain an image having a dynamic range wider than an output of an imaging element (a high dynamic range image) by applying the bracket imaging has been considered. In the imaging of the high dynamic range image, an imaged image with a large exposure amount and an imaged image with a low exposure amount are acquired by the bracket imaging and combined to generate an image having a high dynamic range. In other words, an image component in which gradation on a high luminance side is obtained by holding down an exposure amount and an image component in which gradation on a low luminance side is obtained by increasing an exposure amount are combined. This makes it possible to incorporate gradation information in a wide luminance range, which may be unable to be obtained in one exposure, in an image after the combination. There is also proposed a method of providing, instead of using a bracket imaging function, two kinds of large and small apertures on an imaging element and combining outputs detected in areas of the respective apertures to obtain a high dynamic range image.